Method for oxidizing a substrate surface using oxygen

ABSTRACT

A method for treating a compound semiconductor substrate, in which method in vacuum conditions a surface of an In-containing III-As, III-Sb or III-P substrate is cleaned from amorphous native oxides and after that the cleaned substrate is heated to a temperature of about 250-550° C. and oxidized by introducing oxygen gas onto the surface of the substrate. The invention relates also to a compound semiconductor substrate, and the use of the substrate in a structure of a transistor such as MOSFET.

FIELD OF THE INVENTION

The present invention relates to a method for treating a compoundsemiconductor substrate in order to produce a crystalline oxide layer onthe substrate. The invention also relates to a compound semiconductorsubstrate, and the use of the substrate in a structure of a transistorsuch as a MOSFET, or in a structure of an optoelectronic device.

BACKGROUND OF THE INVENTION

The understanding and development of the oxidized semiconductor surfacesand oxide-semiconductor interfaces are relevant to many technologiessuch as passivation of surface defects of outermost device parts andprocessing of electronics materials as well as manufacturing ofinsulator-semiconductor interfaces for themetal-oxide-semiconductor-field-effect-transistors (MOSFETs). Perhapsevery semiconductor device includes an insulator-semiconductorinterface, MOSFET is a good example. Current MOSFETs, used for examplein microprocessors, are predominantly based on the Si channel andsilicon dioxide (SiO₂) and hafnium dioxide (HfO₂) gate insulators. Thesedevices are facing their fundamental limits as more powerful componentsare developed. III-V compound semiconductors such as InAs, InGaAs, InSband InP are desired channel materials for future MOSFETs, due tosuperior mobilities of electrons in these materials in comparison tosilicon (Si). Therefore, significant efforts have been initiated toproduce gate insulator interfaces of III-V channel layers, which arestable and meet commercial device criteria, as the SiO₂—Si junctionsuccessfully does. However, this great goal, which would lead, forexample, to the increased lifetime of devices and energy savings atservers, has not been yet achieved.

One of the main reasons for this is the presence (or formation) ofnative amorphous III-V surface oxides, which cause Fermi-level pinningvia high density of defect states at the semiconductor-insulatorinterface. So, these native amorphous oxides are detrimental to thetransistors. Therefore, a huge amount of work has been done to find themethod to passivate III-V surfaces against the reaction with oxygen andformation of amorphous oxides. This is however a very challenging tasksince it is difficult to avoid the reaction between III-V semiconductorsurface and oxygen during the growth of interfaces. For example, duringthe insulator layer growth, the III-V surfaces usually react withoxygen. So, it is still not known whether avoiding the oxygen reactionduring interface growth is possible. However, it is well known thatprocessing of the starting III-V surfaces significantly affects theproperties of MOSFETs, and crystalline (or epitaxial) oxide interfacesare highly desired for these devices.

Recently [1], an interesting improvement has been found in theInAs-channel MOSFET of which InAs surface was thermally oxidized in afurnace with an atmospheric pressure conditions. Thetransmission-electron-microscopy image from this interface shows theformation of crystalline islands of InAsO_(x). However, the surface ofthe resulting layer including InAsO_(x) is not long-range ordered andbecomes contaminated in the atmospheric preparation conditions used.

In previous vacuum-based experiments [e.g., Ref. 2], it has been foundthat different surface structures on the starting III-V substrate affectthe oxidation and the properties of the resulting III-V surface oxides.The presented III-V surface oxides are amorphous without long-rangeorder. Therefore, any oxidation of III-V surfaces has been commonlyconsidered to be harmful and tried to be avoided.

To recapitulate, the unsolved problem relating to the use of the III-Vcompound semiconductors in MOSFETs is an amorphous semiconductor-oxideinterface (or the lack of enough crystalline oxide-semiconductorinterface), which causes harmful effects such as Fermi-level pinning, adetrimental leakage currents, and decrease in the carrier mobilities inthe MOSFETs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel method fortreating III-V compound semiconductor substrate in such a way that theabove mentioned problems can be avoided.

It is especially an object of the present invention to provide a methodfor forming a stable and crystalline oxide layer on the III-V compoundsemiconductor substrate, especially on the Indium (In)-containingIII-Arsenic (As), III-Antimony (Sb) or III-phosphorus (P) substrate.

It is especially an object of the present invention to provide a methodfor producing stable and crystalline oxygen-induced III-V semiconductorsurfaces, which method would also be very helpful for producing thecrystalline MOSFET interfaces.

It is also an object of the invention to present a simple process forforming a crystalline long-range ordered oxide layer on the compoundsemiconductor substrate.

In order to achieve the objects mentioned above, the method and thecompound semiconductor substrate according to the present invention arecharacterized in what is defined in the characterizing part of theenclosed independent claims.

Some preferred embodiments according to the invention are disclosed inthe dependent claims presented further below.

Further, the invention relates to the use of the III-V compoundsemiconductor substrate according to the invention in a structure of atransistor, such as in a structure of a MOSFET. The invention alsorelates use of the III-V compound semiconductor substrate according tothe invention in a structure of an optoelectronic device such as a lightemitting diode, photodiode, photocapacitor, photovoltaic cell orsemiconductor based laser.

In the method according to the invention for producing a crystallineoxide layer on an In-containing III-As, III-Sb or III-P compoundsemiconductor substrate, in vacuum conditions

-   -   a surface of an In-containing III-As, III-Sb or III-P substrate        is cleaned from amorphous native oxides, and after that    -   the cleaned substrate is heated to a temperature of about        250-550° C. and oxidized by introducing oxygen gas onto the        surface of the substrate.

Typically, a compound semiconductor substrate according to the presentinvention comprises at least

-   -   an In-containing III-As, III-Sb or III-P base material having a        first side and a second side, and    -   a crystalline (3×1)-O, (2×3)-O, c(4×2)-O, (1×2)-O, (3×1)-SnO,        (3×3)-SnO, or (1×1)-SnO oxide layer being formed on at least a        part of the first side of the base material.

Preferably, the crystalline oxide layer is formed on the base materialof the compound semiconductor substrate by the method according to theinvention.

Surprisingly, it has now been found that the Indium (In)-containingIII-Arsenic (As), III-Antimony (Sb) or III-phosphorus (P) substrate canbe oxidized in a controlled way in vacuum conditions in order to producea stable and crystalline oxide layer on the surface of the substrate,which oxide layer does not further react with oxygen. In other words,the present invention is based on the fact that no attempt is made toavoid the reactions between said III-V material and oxygen as has beendone in the prior art, but the oxidation is carried out in a controlledway in order to produce desired crystal structure on the surface of thecompound semiconductor substrate. The preparation of surface oxidesaccording to the invention requires the vacuum conditions. Therefore,the method presented in Ref. [1] does not provide the crystalline oxidelayers, found here, with the long-range ordered surfaces.

Long-range ordered oxide surface means here a uniform layer which has awell-defined structure over the whole sample surface, providing a clearLEED pattern.

Especially, a family of novel ordered oxide layers on InAs (Indiumarsenide), InGaAs (Indium gallium arsenide), InSb (Indium antimonide),InGaSb (Indium gallium antimonide) and InP (Indium phosphide) surfaceshas now been found. The ordered oxide layer has a crystalline (3×1)structure on the surface of the In-containing III-As and III-Sb surface,and a crystalline (2×3) structure on the InP surface. In addition tothose, an oxide structure of c(4×2) forms on the In-containing III-Assurface, and a (1×2)-O structure on the In-containing III-Sb. The oxidelayers have been identified by using low-energy-electron-diffraction(LEED), scanning-tunneling-microscopy/spectroscopy (STM/STS), as well ascore-level and valence-band photoemission measurements.

In the method for forming a crystalline oxygen-induced In-containingIII-As, III-Sb or III-P semiconductor surface, the formation of theinitial surfaces is crucial. In the method according to the invention,the desired starting surfaces are formed by cleaning the semiconductorsurfaces in vacuum conditions in such a way that the poor (amorphous)native surface oxides and the carbon contamination can be removed fromthe surface of the substrates. The starting InAs and InSb surfacesformed by the cleaning have the c(8×2) reconstruction and the startingInP surface has the (2×4) reconstruction.

During the oxidation method according to the invention, the crystallineoxide layers described above can be formed on the In-containing III-As,III-Sb or III-P substrate. In the method, the temperature of the cleanedsubstrate is raised to a certain level and the oxidation of thesubstrate is carried out in a controlled manner in vacuum conditions byintroducing the oxygen gas onto the surface of the substrate. Theformation of the crystal oxide layer on the surface of the substrate isdependent on the initial surface of the substrate, the temperature andthe oxygen gas pressure during the oxidation, and the oxidation time.

The pressure of the vacuum chamber, in which the method according to theinvention is carried out, is lower than atmospheric pressure i.e. themethod according to the invention is carried out in vacuum conditions.Typically, the base pressures of the vacuum chamber in the methodaccording to the invention are lower than 5×10⁻⁸ mbar. The lower levelof the pressures of the vacuum chamber is dependent on the equipmentused.

The substrate temperature during oxidation is kept at about 250-550° C.in order to produce the crystalline oxide layers onto the surface of thesubstrate. Specifically, a substrate temperature of about 340-400° C.during the oxidation produced an InAs(100)(3×1)-O layer and atemperature of about 340-450° C. produced an InSb(100)(3×1)-O layer. Thesubstrate temperature will be about 450-500° C. for forming thecrystalline (2×3)-O layer on the InP substrate. In the case of InGaAsand InGaSb the temperature range is about 400-550° C. However, it has tobe noted that the above-mentioned temperature ranges are dependent onthe oxygen gas pressure and it may be possible to produce thecrystalline oxide layers also in temperatures close to the definedranges, if the oxygen gas pressure is varied. The limit of inaccuracy ofthe thermometry was ±25° C.

The oxygen gas pressure is preferably between 5×10⁻⁷ and 5×10⁻⁵ mbarduring the oxidation, and the substrate is oxidized preferably for about15-45 minutes and more preferably for about 15-30 minutes for producingthe crystalline oxide layers on the surface of the substrate. Theheating and the oxidation of the substrate can be carried outsimultaneously or the substrate can be heated at the desired temperaturebefore starting the oxidation process. Normally, the heating and oxygenexposure were shut down simultaneously after the oxygen exposures.

The formed oxide layers are crystalline and stable. The thickness of theoxide layer is typically 0.2-1 nm and more typically 0.2-0.5 nm.

In one embodiment according to the invention, the cleaned III-V surfaceis covered by the tin (Sn) layer, of which thickness is 0.5-2 atomiclayer (monolayer). The heating of the Sn-covered surface at 300-550° C.produces the (1×2) Sn-induced structure. The oxidation of thesesurfaces, as described above, produces the crystalline oxide-layerincluding tin. Such SnO-containing layers have the same thickness as the(3×1)-O has, and owns the following structures (long-range ordered):(3×1) and (3×3) on the InAs substrate, (3×1) on the GaAs substrate, and(1×1) on the InP substrate.

The crystalline oxide layer formed in the method according to theinvention will function as a passivating layer, which protects thesemiconductor substrate in the formation of natural amorphous oxides. Inaddition, the resulting oxide layers have no states at the Fermi-level.Therefore, the substrate according to the invention makes possible thedevelopment of more powerful components, for example MOSFETs.Especially, the method and substrate according to the invention can beused in the field of transistors. The invention makes possible the useof In-containing III-As, III-Sb or III-P materials as channel materialsfor future complementary MOS devices.

The crystalline oxide layers according to the invention can also beapplied as a barrier layer with a large energy band gap in devices likelaser diodes and LEDs, and as a part of the passivation of the outermostdevice surfaces. The substrate according to the invention can be used ina structure of an optoelectronic device such as a light emitting diode,photodiode, photocapacitor, photovoltaic cell or semiconductor basedlaser.

Alternatively, the In-containing III-As, III-Sb or III-P substrateaccording to the invention can be a layer, which has been applied ondurable and inexpensive Si substrates. This can be done for example withvarious crystal growth methods. The ultimate goal is that carriers(electrons and holes) move fast in the III-channel layers produced on Sisubstrates.

DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 shows a simplified representation of a cross section of acompound semiconductor substrate according to the invention,

FIG. 2 shows LEED patterns from the InAs(100)c(4×2)-O andInAs(100)(3×1)-O layers. The white squares show the (1×1) unit cells ofthe InAs substrate and the white rectangles show the unit cells of thec(4×2)-O and (3×1)-O layers.

FIGS. 3a, 3b and 3c show (a) an atomic model for the c(8×2) structure ofthe starting InAs surface, (b) an atomic model for the (3×1)-O layer onInAs, and (c) an atomic model for the c(4×2)-O layer on InAs. The O, In,and As atoms are shown with black, white, and gray spheres,respectively.

FIG. 4 shows valence-band photoemissions from the InAs(100)(3×1)-O andc(4×2)-O layers,

FIG. 5 shows core-level photoemission spectra for In 4d and As 3d lines,and

FIG. 6 shows a simplified cross-sectional view of a structure insemiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a simplified representation of a cross section of acompound semiconductor substrate according to the invention. Thesubstrate comprises In-containing III-As, III-Sb or III-P base material1 having a first side and a second side, and a crystalline oxide layer 2being formed on at least a part of the first side of the base material.The base material can be an In-containing III-As, III-Sb or III-Psubstrate or it can be an In-containing III-As, III-Sb or III-P layer onthe surface of a substrate made of some other material, for examplesilicon (Si). The crystalline oxide layer has a structure of (3×1)-O,(2×3)-O, c(4×2)-O, (1×2)-O, (3×1)-SnO, (3×3)-SnO, or (1×1)-SnO dependingon the base material.

Preferably, the crystalline oxide layer 2 is formed on the whole surfaceof the first side of the base material. The oxide layer can comprisesome crystal defects i.e. amorphous regions or other crystallinestructures, but preferably at least 90% of the layer comprises saidcrystalline structure. The valence-band and core-level photoemissionresults indicate that possible defect concentration is smaller than5×10¹¹ defects per cm².

In the method for forming a crystalline oxygen-induced In-containingIII-As, III-Sb or III-P semiconductor surface, the surfaces of the basematerial are first cleaned from the amorphous native surface oxides andthe carbon combination. The cleaning can be carried out by argon ionsputtering and post heating in an ultrahigh vacuum (UHV) chamber. Theargon ion sputtering and post heating in UHV conditions at least to 400°C. leads to well-defined c(8×2) reconstructions on the starting InAs andInSb surfaces and a (2×4) structure on the InP substrate. These startingsurfaces can be obtained also by pure heating in UHV at around 400-550°C. or more to remove a protective As-cap layer produced by an epitaxialgrowth method. In the vacuum conditions, preferably the chamber basepressures lower than 5×10⁻⁸ mbar were used.

Before oxidations, the clean starting surfaces showed (i) sharp c(8×2)or (2×4) low-energy-electron-diffraction (LEED) patterns, (ii) largesmooth areas with terraces usually in the order of 100 nm in diametersize as deduced by scanning tunneling microscopy (STM), and (iii) nooxygen and carbon contamination as deduced by x-ray photoelectronspectroscopy (XPS).

In the oxidation process, oxygen is adsorbed on the c(8×2) or (2×4)surface. FIG. 3a shows the InAs surface with an oxygen atom in theenergetically most favourable atom position. The stability of thisadsorption position, which is located in the second surface layer, i.e.below the surface, is very significant. It is so stable (˜1 eV morestable than the other adsorption positions) because the relativelyelectronegative oxygen is bound to four relatively electropositiveIndium atoms. It is important to note that this kind of adsorption siteis found because the peculiar c(8×2) structure includes a mixed (III-V)first surface layer.

A better understanding of the present invention such as oxidationconditions may be obtained through the following examples which are setforth to illustrate, but are not to be construed to limit of the presentinvention.

The following examples have been carried out using a surface-sciencesystem (Omicron) which includes three different vacuum chambersconnected via gate valves, so that the samples can be transferredbetween chambers without breaking the vacuum conditions. The sampleswere put into the vacuum through the loading chamber. The oxidationswere performed in the preparation chamber, and oxidized surfaces werecharacterized in the analysis chamber. The same surface cleaning andoxide preparations were done also in a separate vacuum system at theMAX-lab using the beamline 41 at the synchrotron ring MAX-I.

Oxidation experiments were performed in a vacuum chamber using molecularoxygen gas with a leak valve via which O₂ gas was injected into thechamber. The O₂ pressure was controlled to be between about 5×10⁻⁷ and5×10⁻⁵ mbar during oxidation experiments. The sample surface was facedtoward the leak valve at a distance of about 15 cm. The oxidation timeof the substrates heated simultaneously was about 15-30 minutes. Theheating and oxygen exposures were shut down simultaneously after theoxygen exposures. The substrate temperature during the above mentionedoxidation conditions affected the formed crystal structure of theoxidized semiconductor, preferably as follows:

-   -   The temperature about 340-400° C. produced an InAs(100)(3×1)-O        layer    -   The temperature about 400-450° C. produced an InAs(100)c(4×2)-O        layer    -   The temperature about 340-450° C. produced an InSb(100)(3×1)-O        layer    -   The temperature about 340-450° C. produced an InSb(100)(1×2)-O        layer    -   The temperature about 450-550° C. produced an        InGaAs(100)c(4×2)-O layer,        which clearly indicates that InGaAs(100)(3×1)-O layer is formed        below 550° C. if the amount of Indium is high enough in the        surface layer. The calculations predict the formation of        InGaAs(100)(3×1)-O when the Indium amount increases in a surface        layer of InGaAs. The temperature range in the case of InGaSb is        similar as InGaAs.

For the oxide layers including tin (Sn), the substrate temperatureduring the above mentioned oxidation conditions affected the formedcrystal structure of the oxidized semiconductor, preferably as follows:

-   -   The temperature about 370-400° C. produced an InAs(100)(3×1)-SnO        layer    -   The temperature about 400-450° C. produced an InAs(100)(3×3)-SnO        layer    -   The temperature about 370-450° C. produced an InP(100)(1×1)-SnO        layer    -   The temperature about 450-550° C. produced an GaAs(100)(3×1)-SnO        layer.

The examples of the oxidation process of the InAs and InSb substratesare shown on the following Tables 1 and 2. The substrates are heated tothe desired temperature before starting the oxidation. The formedcrystalline structure of the (3×1)-O layer was detected by thelow-energy-electron-diffraction (LEED) measurement. Table 1 presents theoxidation of InAs substrate with the different oxidation times. Theoxygen gas pressure was 4×10⁻⁶ mbar during oxidation.

TABLE 1 Oxidation T (° C.) t (min) LEED I 374 5 c(4x2) II 376 15 (3x1) +weak c(4x2) III 375 20 (3x1) + weak c(4x2) IV 374 30 (3x1) V 373 45weaker (3x1)

Table 2 presents the oxidation of InSb substrate with the differentoxidation times, the oxygen gas pressure was 3-4×10⁻⁶ mbar.

TABLE 2 Oxidation T (° C.) t (min) LEED I 350 5 (1x2) with splitted x2spots II 349 10 (1x2) with splitted x2 spots III 353 15 (3x1) + weak(1x2) with splitted x2 spots IV 351 20 (3x1) V 351 30 (3x1)

There is also an alternative embodiment to produce a (3×1)-O layer onthe In-containing III-As or III-Sb substrate. Namely, theInAs(100)c(4×2)-O layer was prepared first on the substrate by using atemperature of 400-450° C., and then the substrate was taken out fromthe vacuum chamber into air where it was kept about 30 min. After thatthe sample was transferred back into the vacuum conditions and heated toabout 400-450° C. for 20 min. That also led to the formation of (3×1)-O.This shows that the starting InAs(100)c(4×2)-O layer can “catalyze” theformation of (3×1)-O since the same air-exposure of clean InAs surfacedoes not produce the (3×1)-O. In FIG. 3c is shown an atomic model forc(4×2)-O layer on InAs structure.

FIG. 2 shows the LEED intensity patterns from the c(4×2)-O and (3×1)-Olayers on the InAs substrate. It is worth noting that the pattern fromthe InSb(100)(3×1)-O layer was similar to that in FIG. 2. First of all,the sharp intensity spots with low background intensity reveal that bothoxide layers are well-ordered or crystalline. Second, the additionalintensity spots inside the 1×1 unit cell, which is shown with a whitesquare in FIG. 2, reveal that the oxide layer includes (3×1) crystallattice, which is shown with the white rectangle.

By comparing the different measurements with ab initio calculations, apreliminary atomic model for the crystal structure of (3×1)-O layers isproposed in FIG. 3 b.

FIG. 4 shows valence-band photoemissions from the InAs(100)(3×1)-O andc(4×2)-O layers. The measurements show that these interfaces do notinclude metallic electronic states around the Fermi energy. Moreover, itis important to note that the band bending, which is more sensitive todefects, did not occur as compared to the clean InAs(100)c(8×2),supporting the absence of pinning. These results indicate that theharmful Fermi-level pinning can be avoided in MOSFETs. Also scanningtunneling spectroscopy (STS) measurements show that these crystallineoxide layers do not cause harmful electronic states in the band-gap ofInAs. The valence band maximum difference (or offset) between the InAssubstrate and the (3×1)-O layer is estimated in FIG. 4 using adifference spectrum between the InAs(100)(3×1)-O and InAs(100)c(4×2)-Ospectra, in which the InAs band edge is basically removed. Thisdifference spectrum gives an estimation for the (3×1)-O valence bandedge: it has a 1.0-1.4 eV higher binding energy than the InAs one, whichmight be high enough to provide an energy barrier for electric carriersin the valence band of MOSFETs. For the reference, the Fermi-levelemission was measured from an air-exposed InAs(100)c(8×2) surface, whichwas oxidized and showed a poor (1×1) LEED. That spectrum showed anemission at the Fermi-level, as expected to arise from defect states ofsuch an amorphous surface.

FIG. 5 shows core-level photoelectron spectra for the In 4d and As 3dlines. The features (peaks or shoulders) at the low kinetic energy sidedemonstrate the presence of oxygen and the formation of the oxide layer.

The interface stability that is an important property for theapplications was tested as follows. The passivity of theInAs(100)(3×1)-O and InAs(100)c(4×2)-O surfaces were tested by takingthe samples into the atmosphere for 0.5-1 hours. After that they weretransferred back into the vacuum chamber and characterized by LEED andSTM as a function of the post heating time. After one hour air exposure,the InAs(100)(3×1)-O sample produced, without any post heating, a (3×1)pattern. Naturally, the obtained pattern was weaker than the (3×1)pattern before air exposure, but the pattern became clear (3×1) byheating the substrate to 400° C. for 30 min. These LEED findings aresupported by the STM from the InAs(100)(3×1)-O surface after 0.5-hourair exposure and post heating at 400° C. One should note that the sameair exposure and post heating procedure of the clean InAs(100)c(8×2)surface led to a poor (1×1) LEED without any clear superstructure. Theseresults indeed show that the InAs(100)(3×1)-O layer is stable againstrather strong air and temperature loadings. This is an importantproperty, which allows for example the transfer of the (3×1)-O samplevia air into another chamber to grow for example a SiO₂ top insulatorlayer. The stability also indicates that the (3×1)-O layer does notbreak during the deposition of a top insulator layer. It is worth notingthat the atomic layer deposition (ALD) of the gate insulators onInAs(100) and InGaAs(100) is usually performed at substrate temperaturesof 250-350° C., at which the InAs(100)(3×1)-O was clearly stable. Thesame passivation tests for the InAs(100)c(4×2)-O showed a poor (1×1)after air exposure, but after post heating to about 400° C., a (3×1)LEED appeared. This indicates that the c(4×2)-O structure might“catalyze” the formation of the (3×1) layer since we did not observe thesame formation after the air exposure of the clean InAs(100)c(8×2)surface.

The following example describes the oxidation process according to theinvention by using the InGaAs substrate.

The surface of the GaAs(100) substrate wafer (about 0.5 mm thick) wascleaned from the carbon contamination and amorphous surface oxides withAr-ion sputtering and heating in an ultrahigh vacuum (UHV) chamber.During the 30 min raster sputtering, the Ar-gas pressure was 1-3×10⁻⁶mbar, the sputtering voltage was 0.7 kV and the current 10 mA, and theGaAs substrate temperature was about 400° C. After this sputtering, theGaAs substrate was heated to about 570° C. in a vacuum smaller than1×10⁻⁹ mbar for 30 min. Six such sputtering-postheating cycles produceda clean and smooth surface, which had the (6×6) structure as deduced byLEED and STM. To prepare an InGaAs surface, 1-2 monolayers (ML) ofindium were deposited on the clean GaAs(100)(6×6) substrate surface andthe sample was heated to 500-550° C. This produced the InGaAs(100)c(8×2)surface. This clean well-defined InGaAs(100)c(8×2) surface was oxidized,resulting in the c(4×2)-O layer similar to the InAs(100)c(4×2)-O case,as follows: First the substrate temperature was increased to about 550°C. in 15 min in vacuum conditions. Then the leak valve of the oxygen(O₂) gas line, connected to the vacuum chamber, was opened and theoxygen pressure was increased to 3-4×10⁻⁶ mbar. After the 15 minoxidation at the oxygen pressure of 3-4×10⁻⁶ mbar and 550° C. (InGaAs),the heating was closed 10 s before closing the oxygen flow. The c(4×2)crystal structure was determined by LEED. The formation of the c(4×2)-Olayer in the InGaAs surface clearly indicates that the (3×1)-O layerforms also in InGaAs, like in the InAs case, if the amount of Indium(In) is increased in the InGaAs surface layer.

On the basis of the preparation conditions of InAs(100)c(4×2)-O and-(3×1)-O, in which the (3×1)-O is always formed at a lower temperaturethan c(4×2)-O, it can be concluded that the (3×1)-O forms on InGaAs attemperatures lower than 550° C.

The oxidation of the InP substrate has been carried out as is describedin the following examples.

The surface of the InP(100) substrate wafer (about 0.5 mm thick) wascleaned from the carbon contamination and amorphous surface oxides withAr-ion sputtering and heating in ultrahigh vacuum (UHV) chamber. Duringthe 30 minutes raster sputtering, the Ar-gas pressure was 1-3×10⁻⁶ mbar,the sputtering voltage was 0.7 kV and the current 10 mA, and the InPsubstrate temperature was about 370° C. After the sputtering, the InPsubstrate was heated to about 470° C. in a vacuum smaller than 1×10⁻⁹mbar for 30 min. Three such sputtering-postheating cycles produced cleanand smooth surface, which had the mixed-dimer (2×4) structure as deducedby LEED and STM. This clean well-defined InP(100)(2×4) surface wasoxidized, resulting in the (2×3)-O layer, as follows: first the InPsubstrate temperature was increased to about 480° C. in 15 min in vacuumconditions. Then the leak valve of the oxygen (O₂) gas line, connectedto the vacuum chamber, was opened and the oxygen pressure was increasedto 3-4×10⁻⁶ mbar. After the 15 min oxidation at the oxygen pressure of3-4×10⁻⁶ mbar and the temperature of about 480° C. (InP), the heatingwas closed 10 s before closing the oxygen flow.

The same (2×3)-O layer was also obtained by the 15 min oxidation at theoxygen pressure of 7-8×10⁻⁷ mbar and the temperature of about 460° C.(InP). The (2×3) crystal structure was determined by LEED and STM. X-rayphotoelectron spectroscopy (XPS) revealed three peaks for the O1semission at 528, 531, and 537 eV binding energies. Also the P 2pemission shows two oxygen-related peaks at 133 and 138 eV.

Table 3 presents other examples of the oxidation of InP substrate. Theoxidation of the clean InP(100)(2×4) surface was carried out at about450° C. by changing the oxidation time (the resistive heating voltageand current were 13.0 V and 2.3 A in all the tests I-V). The oxygenpressure was between 5×10⁻⁷ and 5×10⁻⁵ mbar in all cases. The results inTable 3 indicate that 450° C. is a too low temperature for obtainingpure (2×3)-O. The presence of (2×4) in all the cases indicates thepresence of some clean InP(100)(2×4) areas on the surfaces.

TABLE 3 Oxidation T (° C.) t (min) LEED I 453 5 weaker (2x4) than theclean surface (2x4) II 452 10 weak (2x4) + weak (2x3)-O III 455 15(2x3)-O + weak (2x4) IV 450 20 (2x3)-O + weak (2x4) V 453 30 (2x3)-O +weak (2x4)

The oxidation of Sn-covered surfaces was performed as follows. Aftercleaning the InAs substrate surface, as described above, 0.5-2.0 ML oftin was deposited onto the cleaned InAs(100)c(8×2) substrate surface atroom temperature. This Sn-covered surface was then heated in vacuum at350-420° C. for 15-30 min, which provided the Sn-induced (1×2)reconstruction, as deduced by LEED and STM. After that the sample wastransferred in the oxidation chamber, and its temperature was risen upto 420° C. Then the oxygen (O₂) gas was introduced into the chamber viathe leak valve. The oxidation in 3-4×10⁻⁶ mbar for 10 min produced theInAs(100)(3×3)-SnO surface layer. Decreasing the substrate temperatureduring the oxidation and/or increasing the oxygen exposure produced theInAs(100)(3×1)-SnO.

The electronic structure of an insulator-semiconductor interface has asignificant role in the MOSFET transistor applications. The substrateaccording to the invention, comprising In-containing III-As, III-Sb orIII-P base material and a crystalline oxide layer such as (3×1)-O beingformed on the In-containing III-As or III-Sb base material or (2×3)-Obeing formed on the In-containing III-P base material, can be used inthe insulator-semiconductor interface in the MOSFET transistors. Theexample of structure of the MOSFET transistor comprising a compoundsemiconductor substrate according to the invention is presented in FIG.6. It comprises the In-containing III-As, III-Sb or III-P channel layer1 such as InAs, InGaAs, InSb, InGaSb or III-P, and the crystalline oxidelayer 2 formed on the surface of the channel layer. The second insulatorlayer 3, which can be for example SiO₂ or Al₂O₃, is formed on thesurface of the oxide layer. The structure also comprises a gate metal 4,a source contact layer 5, a source metal 6 which can be for exampleAuGeNi, a drain contact layer 7 and a drain metal 8 like AuGeNi. Thecrystalline oxide layer according to the invention serves as a barrierlayer for electrons and holes in the In-containing channel so that theelectric carriers do not leak toward the gate oxide. The crystallineoxide layer acts as an important part of the gate insulator stack.

The invention is not restricted to the examples of the abovedescription, but it can be modified within the scope of the inventiveidea presented in the claims.

REFERENCES

-   [1] H. Ko, K. Takei, R. Kapadia, S. Chuang, H. Fang, P. W. Leu, K.    Ganapathi, E. Plis, H. S. Kim, S. Y. Chen, M. Madsen, A. C.    Ford, Y. L. Chueh, S. Krishna, S. Salahuddin, and A. Javey A:    Ultrathin compound semiconductor on insulator layers for    high-performance nanoscale transistors. Nature Vol. 468: p. 286    (2010).-   [2] G. Chen, S. B. Visbeck, D. C. Law, and R. F. Hicks:    Structure-sensitive oxidation of the indium phosphide (001) surface.    Journal of Applied Physics Vol. 91: p. 9362 (2002).

The invention claimed is:
 1. A method for producing a crystalline oxidelayer on an In-containing III-As, III-Sb or III-P compound semiconductorsubstrate, comprising in vacuum conditions providing an In-containingIII-As, III-Sb or III-P substrate having a substrate surface which isclean from amorphous native oxides, and heating said substrate having aclean surface to a temperature of about 250-550° C., and oxidizing it atsaid temperature of about 250-550° C. by introducing oxygen gas onto thesurface of the substrate, wherein said method produces a compoundsemiconductor substrate comprising at least an In-containing III-As,III-Sb or III-P base material having a first side and a second side, anda crystalline (3×1)-O, c(4×2)-O, (1×2)-O, (2×3)-O, (3×1)-SnO, (3×3)-SnO,or (1×1)-SnO oxide layer being formed on at least a part of the firstside of the base material.
 2. The method according to claim 1, whereinthe In-containing III-As, III-Sb or III-P substrate is made of InAs,InSb, InP, InGaAs or InGaSb.
 3. The method according to claim 1, whereinthe substrate is cleaned by argon-ion sputtering and post heating inultra-high-vacuum (UHV) conditions at least to 400° C., or by pureheating in UHV at around 400-550° C.
 4. The method according to claim 1,wherein the cleaned substrate is covered by a tin (Sn) layer.
 5. Themethod according to claim 1, wherein the cleaned In-containing III-Assubstrate is heated to a temperature of about 340-400° C.
 6. The methodaccording to claim 1, wherein the cleaned In-containing III-Sb substrateis heated to a temperature of about 340-450° C.
 7. The method accordingto claim 1, wherein the cleaned In-containing III-P substrate is heatedto a temperature of about 450-500° C.
 8. The method according to claim1, wherein the heating and the oxidation of the substrate are carriedout simultaneously.
 9. The method according to claim 1, wherein thesubstrate is oxidized for 15 to 45 minutes.
 10. The method according toclaim 9, wherein the substrate is oxidized for 15 to 30 minutes.
 11. Themethod according to claim 1, wherein the oxygen gas pressure is between5×10⁻⁷ and 5×10⁻⁵ mbar.